Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In an application such as an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly variable. This variable frequency power is converted to constant frequency power using a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200 VAC power at 400 hertz. Such known converters are controlled by a generator/converter control unit (GCCU).
In such a power system, the generator is typically connected to the converter through a feeder circuit. Similarly, the output of the converter is connected through a feeder circuit to the aircraft power bus. With known 400 hertz electric power generating systems, feeder faults are detected by measuring the current into and out of each feeder. If there is a feeder fault, either phase-to-phase or to aircraft ground, then the current into a feeder will not equal the current out of that feeder. Current transformers are most commonly used in the detection of such feeder faults. Current transformers are quite accurate when sensing high frequency AC power, such as for example, 400 hertz, so that even high impedance feeder faults can be reliably detected.
In order to provide aircraft engine starting, certain known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motive power to start the engine. To minimize the size and weight of such start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU for the start control. In such VSCF-based electric power systems, the power frequency passed between the VSCF system and the starter is zero hertz when the engine start is initiated, and gradually increases as the engine accelerates. Because current transformer inaccuracy increases as the sensed frequency decreases, the minimum feeder fault current that can be accurately detected by traditional techniques increases with decreasing current frequency. Below a minimum frequency, current transformers saturate and feeder faults will be undetected.
The present invention is intended to overcome one or more of the problems as set forth above.